This study investigated the ability of normal subjects and glaucoma patients to judge the temporal order of small, foveally presented stimuli. Two basic stimulus paradigms were used: Inter-Ocular Temporal Asynchrony (IOTA), and Temporal Order Discrimination (TOD).
IOTA, a simple foveal dichoptic temporal order matching technique, was designed to investigate whether the visual system in glaucoma is characterised by interocular asymmetry of visual latency. Preliminary testing showed that IOTA was affected by monocular luminance attenuation using ND filters in normal subjects. Glaucoma subjects showed significant abnormalities on IOTA testing which correlated strongly with asymmetry of optic disc appearance, and less strongly with asymmetry of visual field indices. This is the first perceptual evidence of alterations in visual latency in glaucoma patients, and is in accord with previous findings that glaucoma causes increased latency in visually evoked potential responses.
TOD was a monocular foveal test, measuring the smallest interstimulus interval required by a subject to distinguish the temporal order of two spatially proximal stimuli. As there had been little previous research in normal subjects on the effects of stimulus duration, contrast and temporal profile on TOD, these parameters were investigated for a small number of young, visually normal subjects. For constant contrast stimuli with rectangle temporal profiles, TOD thresholds depended on stimulus duration, performance being optimal for durations around 10 to 100 ms, and thresholds increasing for longer and briefer stimulus durations. For stimuli set at 4 times contrast threshold, TOD thresholds were constant for brief durations, but increased (i.e. became poorer) for longer durations. This duration-dependent increase in TOD thresholds was more marked for stimuli with Gaussian temporal profiles than for rectangular profile stimuli, suggesting that the high frequency information contained in the edges of rectangular pulses aided TOD judgments. TOD thresholds improved with increasing stimulus contrast, although the degree of improvement varied with stimulus duration, and between subjects.
TOD performance was measured in glaucoma patients, age-matched old normal subjects, and young normal subjects. Stimuli with Gaussian temporal envelopes were used, and contrasts set at 4 times individual subject's contrast threshold so that all stimuli would be equally visible.
Stimulus durations were varied over a large range (pulse standard deviations of 16, 64, and 256 ms). Older subjects showed increased TOD thresholds (i.e. poorer performance) compared to young normal subjects, and this was true for all stimulus durations. This result cannot be explained in terms of increased sluggishness of the aging visual system, but suggests that TOD processing of high and low temporal frequency information is impaired in older observers.
The TOD thresholds of glaucoma patients did not differ significantly from路 those of age-matched normals for all durations tested, despite the fact that glaucoma subjects had, on average, significantly poorer foveal contrast detection thresholds, greater optic disc cupping and a higher degree of visual field loss than for age-matched normals. For clinical testing of foveal function in glaucoma, contrast threshold measurement is of greater diagnostic value than TOD measurement.
The subjects in each of the three groups were tested with MOTA (Monocular Temporal Asynchrony), a "method of adjustment" measure of temporal order matching ability. This technique was trialed because its speed of measurement offered an advantage over TOD in clinical testing.
However MOTA showed poor potential as a substitute for TOD, correlating only weakly with TOD for the three groups, and being unable to distinguish glaucoma from age-matched normal subjects.
In summary, glaucoma appears to cause interocular asymmetry of perceived latency, but does not affect precision of temporal order judgments, even in patients who show elevated foveal contrast thresholds
